1. Field of the Invention
The invention relates to an overvoltage protection means having a first electrode, having a second electrode, with a breakdown spark gap which is formed between the two electrodes, and with a housing which holds the electrodes, when the breakdown spark gap is ignited, an arc forming between the two electrodes within a discharge space which connects the two electrodes.
2. Description of Related Art
Electrical, but especially electronic measurement, control and switching circuits, mainly also telecommunications equipment and systems, are sensitive to transient overvoltages, as can occur especially by atmospheric discharges, but also by switching operations or short circuits in power supply grids. This sensitivity has increased to the extent that electronic components, especially transistors and thyristors, are being used; in particular, increasingly used integrated circuits are highly endangered by transient overvoltages.
Electrical circuits work with the voltage specified for them, the rated voltage (generally≈line voltage), normally without interference. This does not apply when overvoltages occur. Overvoltages are all voltages which are above the upper tolerance limit of the rated voltage. They also include mainly transient overvoltages which can occur due to atmospheric discharges, but also due to switching operations or short circuits in power supply grids, and can be metallically, inductively or capacitively coupled into electrical circuits. Overvoltage protection means have been developed and have been known for more than 20 years to protect electrical or electronic circuits, especially electronic measurement, control and switching circuits, mainly also telecommunications equipment and systems wherever they are used against transient overvoltages wherever they are used.
An important component of an overvoltage protection means of the type under consideration here is at least one spark gap which responds at a certain overvoltage, the sparkover voltage, and thus, prevents overvoltages which are larger than the sparkover voltage of the spark gap from occurring in the circuit which is protected by an overvoltage protection means.
It was stated at the beginning that the overvoltage protection means in accordance with the invention has two electrodes and a breakdown spark gap which is formed between the two electrodes. In practice, these breakdown spark gaps are often also called air breakdown spark gaps, within the framework of the invention, a breakdown spark gap also meaning an air breakdown spark gap. However, here, besides air, another gas can also be present between the electrodes. The region of the overvoltage protection means in which the arc forms when the breakdown spark gas ignites, is hereinafter called the discharge space. It is generally the space between the two electrodes.
In addition to overvoltage protection means with a breakdown spark gap, there are also overvoltage protection means with a flashover spark gap in which a creeping discharge occurs when it responds.
Overvoltage protection means with a breakdown spark gap as compared to overvoltage protection means with a flashover spark gap have the advantage of higher surge current-carrying capacity, but the disadvantage of a higher and also not especially constant sparkover voltage. Therefore, different overvoltage protection means with a breakdown spark gap have already been suggested and have been improved with respect to the sparkover voltage. Here, in the area of the electrodes or the breakdown spark gap which acts between the electrodes, ignition aids have been implemented in different ways, for example, such that, between the electrodes, there has been at least one ignition aid which triggers a creeping discharge and which projects at least partially into the breakdown spark gap, which is made in the manner of a crosspiece and which is made of plastic (compare, for example, German Patent Application DE 41 41 681 A1 or German Patent Application DE 44 02 615 A1 and corresponding U.S. Pat. No. 5,604,400).
The aforementioned ignition aids, which are provided in the known overvoltage protection means, can likewise be called “passive ignition aids.” The are referred to as “passive ignition aids” because they do not respond “actively” themselves, but respond only by an overvoltage which occurs on the main electrodes.
German Patent Application DE 198 03 636 A1 and corresponding U.S. Pat. No. 6,111,740 likewise disclose an overvoltage protection means having two electrodes, with a breakdown spark gap which acts between the two electrodes, and an ignition aid. In this known overvoltage protection means, the ignition aid is made as an “active ignition aid,” in contrast to the above described ignition aids which trigger a creeping discharge, specifically in that in addition to the two electrodes, called the main electrodes there, there are two more ignition electrodes. These two ignition electrodes form a second breakdown spark gap which is used as an ignition spark gap. In this known overvoltage protection means, the ignition aid includes not only the ignition spark gap, but also an ignition circuit with an ignition switching device. When there is an overvoltage on this known overvoltage protection means, the ignition circuit with the ignition switching device provides for response of the ignition spark gap. The ignition spark gap and the two ignition electrodes are arranged with respect to the two main electrodes such that, because the ignition spark gap has responded, the breakdown spark gap between the two main electrodes, called the main spark gap, responds. Response of the ignition spark gap leads to ionization of the air present in the breakdown spark gap so that, after response of the ignition spark gap, the breakdown spark gap also suddenly responds between the two main electrodes, therefore the main spark gap.
In the known, above described embodiments of overvoltage protection means with ignition aids, the ignition aids lead to an improved, specifically lower and more constant sparkover voltage.
In overvoltage protection means of the type under consideration—with or without using an ignition aid—when the breakdown spark gap ignites, the resulting are forms a low-impedance connection between the two electrodes. First of all, the lightning-stroke current to be diverted flows intentionally by way of this low-impedance connection. However, when the line voltage is present, then an unwanted line follow current follows by way of the low-impedance connection of the overvoltage protection means, so that an effort is made to extinguish the arc as quickly as possible after the completed diversion process. One possibility for achieving this object is to increase the arc length, and thus, the arc voltage.
One possibility for extinguishing an arc after a diversion process, specifically increasing the arc length and thus the arc voltage, is implemented in the overvoltage protection as is known from the above mentioned German Patent Application DE 44 02 615 A1 and corresponding U.S. Pat. No. 5,604,400. The overvoltage protection means known from German Patent Application DE 44 02 615 A1 and corresponding U.S. Pat. No. 5,604,400 has two narrow electrodes which are each angularly shaped and have an arcing horn and a connecting leg angled off from it. In addition, the arcing horns of the electrodes are provided with a hole in their areas bordering the connecting legs. The holes provided in the arcing horns of the electrodes provide for the resulting arc “being set into motion” by a thermal pressure effect at the instant of response of the overvoltage protection element, therefore of ignition, and migrating away from its origin. Since the arcing horns of the electrodes are arranged in a V-shape relative to one another, the segment to be bridged by the arc is thus enlarged when the arc migrates out, by which the arc voltage also rises. However, the disadvantage here is that, to achieve the desired increase of arc length, the geometrical dimension of the electrodes must be correspondingly large, so that the overvoltage protection means is also tied altogether to certain geometrical constraints.
Another possibility for extinguishing the arc after the diversion process consists in cooling the arc by the cooling action of insulation walls and the use of insulators which release gas. Here, a strong flow of the extinguishing gas is necessary; this requires high construction effort.
Moreover, it is possible to increase the arc voltage by increasing the pressure. To do this, German Patent DE 196 04 947 C1 proposes selecting the volume in the housing interior such that the arc causes a pressure increase to a multiple of atmospheric pressure. Here the increase in the follow current extinguishing capacity is achieved by a pressure-dependent effect on the arc field strength. So that this overvoltage protection means works reliably, a very pressure-resistant housing is, however, necessary, on the one hand, the level of the line voltage must be known relatively exactly to be able to design the volume in the housing interior accordingly, on the other hand.
If the arc is extinguished in overvoltage protection means of the type under consideration, first of all, the low-impedance connection between the two electrodes is interrupted, the space between the two electrodes, i.e., the discharge space, is however still almost completely filled with a conductive plasma. The plasma which is present reduces the sparkover voltage between the two electrodes such that, at the prevailing line voltage, re-ignition of the breakdown spark gap can occur. This problem occurs especially when the overvoltage protection means has an encapsulated or half-open housing, since then cooling or volatilization of the plasma is prevented by the essentially closed housing.
To prevent re-ignition of the overvoltage protection means, i.e., the breakdown spark gap, in the past various measures were taken to drive the ionized gas cloud away from the ignition electrodes or to cool it. To do this, structurally complex labyrinths and cooling bodies are used, which make production of the overvoltage protection means more expensive.